The present invention relates to a brake control method and a brake control device, which are used for a braking device provided to a vehicle or the like, the braking device having a function of applying a brake force to a wheel equipped with a tire to put a brake thereon while adjusting the brake force, and more particularly, to a brake control method and a brake control device for adjusting a brake force in accordance with a contact length of a tire and for reducing a braking distance.
When a brake is put on a vehicle travelling on an extremely slippery road surface, there is a fear that wheels lock, making it impossible to steer the vehicle. When the wheels lock, it is impossible to perform a risk aversion operation with respect to an obstacle ahead, leading to a very dangerous situation in terms of safety. Therefore, at present, an anti-lock brake system (hereinafter, referred to as ABS) is provided to most vehicles.
Meanwhile, a friction force of a tire increases along with an increase of a slip ratio as long as the slip ratio falls within a small range. However, when the slip ratio increases to a certain extent, the friction force reaches a maximum value thereof. After that, when the slip ratio further increases, the friction force decreases, leading to a situation in the end where the wheels lock. In this case, however, a value of the slip ratio which causes the wheels to lock varies depending on a tire provided to the vehicle, and road conditions such as a wet road surface or a road surface covered with snow. For this reason, the ABS has a control program set to repeat the control of a braking pressure (for example, to reduce, to maintain, or to increase the pressure) when the slip ratio has reached a predetermined threshold value which is determined in advance with a margin, so as to prevent the wheels from locking.
However, the conventional ABS performs control to prevent the wheels from locking, based on a control program in which a threshold value is set with a certain margin. Accordingly, a braking distance may be increased by an amount corresponding to the allowance.